Light emitting diodes (LEDs) are fabricated from compound semiconductor materials, which have the characteristic of emitting light when biased with a forward current. LEDs are widely used as indicators or displays in various types of appliances. Historically, LEDs emitted a relatively low level of light compared to other light sources and were suitable for indoor applications only.
Recent advances in compound semiconductor materials research have yielded new LEDs, which emit very high levels of light. Examples of these new LED materials are Aluminum Indium Gallium Phosphide (AlInGaP) and Indium Gallium Nitride (InGaN). These high brightness LEDs have given rise to new LED devices suitable for applications in areas such as outdoor video displays, automotive signals, traffic signals and illumination.
The high output achieved with these devices is the result of efficient semiconductor materials and of driving the LEDs at very high forward currents. Drive currents in the hundreds or thousands of milliamperes (mA) are often utilized. Unfortunately, such high drive currents produce excessive heat. Since the efficiency of an LED decreases at these high temperatures, light output starts to drop. In addition, the packaging of the devices starts to break down due to prolonged exposure to the elevated temperatures. Such packaging failures limit useful life of the device. A number of device packages have been proposed; however, none of these provide sufficient heat dissipation for the current generation of high-power LEDs.